In this specification the term “light” will be used in the sense that it is used in optical systems to mean not just visible light, but also electromagnetic radiation having a wavelength outside that of the visible range.
Design criteria for optical components are moving towards smaller size and greater functionality. In general, this has led towards greater integration of components. In particular, increasingly the functionality of optical chips made in materials such as silicon, silicon dioxide, indium phosphide and gallium arsenide has enabled the creation of multi-functional and small devices such as optical transmitters and modulators.
However, there remain disadvantages of complete chip integration: the cost of yield impairment, significant optical losses and the reduced manufacturing flexibility of one chip one product.
At the other end of the scale, the cost and complexity of assembling, aligning and fixing multiple optical components in a complex optical device remains a challenge. The accurate alignment of optics in multi-component modules may presently take many hours per module.
Optical transmitters and receivers for high data rates have been enabled by coherent technologies. Today 100 Gb/s and/or higher line rates utilise higher order amplitude and phase modulation, polarization multiplexing, coherent detection and sophisticated digital signal processing techniques. In a typical Dual Polarization Quadrature Phase Shift Key (DP-QPSK) implementation, for example, a 100 Gb/s line rate can be generated from a 25 GHz clock/data rate using high speed digital to analogue converters (DAC) and a pair of QPSK Mach Zehnder modulators (MZM). The data is effectively encoded on two phase and two polarization states of an input laser carrier. Similarly, the encoded data can be extracted at the receiver by polarisation de-multiplexing & coherent detection techniques. In Quadrature Amplitude Modulation (QAM) implementations, both amplitude and phase states with polarization are utilised to increase the line rate.
Transmitters and receivers are typically co-located in a system and are often co-packaged to make transceivers. A laser carrier source can be either integrated into the transmitter (monolithically or as a separate chip) or can be located nearby in an independently packaged device configured to supply an input light signal to the system. In a coherent detection scheme the coherent receiver generally requires the input of a local oscillator (LO) laser reference to extract the QPSK encoded data. The LO source can be derived from an independent laser but for the purposes of efficiency, is typically derived by splitting the input light signal that is provided by the laser source.
A basic coherent transmitter consists of a tunable laser source (carrier) providing an input light signal, Mach-Zehnder interferometers (modulators) and a polarisation combiner to create a composite signal for transmission. The transmitter may be a III-V chip with varying levels of functional integration. The tunable laser may also provide a LO signal for an adjacent receiver operating at the same wavelength or a close wavelength to the transmitter.
In order to create an efficient transmitter module from discrete parts including modulator chips, input fibres and an independent laser, the input light signal is split to supply an optical signal to the receiver LO as well as to the modulators. In addition, an output signal from the modulator chip has to be transformed into its orthogonal polarisation and then the orthogonally polarised light signals output from the transmitter must be combined to give the output signal.
A basic coherent receiver configuration consists of a LO, polarization splitter, a pair of 3 decibel (dB) hybrid mixers and an array of balanced detectors. The receiver may be a III-V chip with varying levels of functional integration.
In addition to the basic functionality mentioned above, both the transmitter and receiver typically require a means to adjust signal levels and a means to control signal levels in the form of a variable optical attenuators and optical monitors respectively.
A coherent receiver where many functions are contained on one chip is described in U.S. Pat. No. 8,526,102. Such a large and complex chip suffers from the disadvantages described herein.
Similarly, where the level of chip integration is limited then the remaining functions are provided by conventional optics where each component in the optical train is put in place and optically aligned individually.